Make no_std build work with serde feature enabled

[aeryz]

when serde feature is enabled in a no_std build, it fails due to the references to std. This PR makes serde feature compatible with no_std.

Note that thiserror is completely removed because it doesn't support no_std.

[ralexstokes]

a few questions but generally I'm open to this

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +29.64 🎉

Comparison is base (2300ff8) 44.28% compared to head (51f852a) 73.93%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #58       +/-   ##
===========================================
+ Coverage   44.28%   73.93%   +29.64%     
===========================================
  Files          20       18        -2     
  Lines        1409      844      -565     
===========================================
  Hits          624      624               
+ Misses        785      220      -565     

Impacted Files	Coverage Δ
ssz-rs/src/lib.rs	100.00% <ø> (ø)
ssz-rs/src/list.rs	75.00% <ø> (ø)
ssz-rs/src/serde.rs	70.00% <0.00%> (-30.00%)	⬇️
ssz-rs/src/vector.rs	83.33% <ø> (ø)

... and 2 files with indirect coverage changes

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[ralexstokes]

hey sorry for the delay here, there is an audit underway and I was trying to minimize how much main differed from the audit target but I'm losing that battle

this looks good!